1. Field of the Invention
The present invention relates to a condenser microphone and a method for manufacturing a substrate used for the condenser microphone.
2. Description of the Related Art
A related art will be described below taking the case of what is called a back electret type electret condenser microphone (referred to as an ECM below). FIG. 1 shows a sectional configuration of an ECM in accordance with the related technique, which is described in Japanese Patent Application Laid Open No. 2003-153392. In FIG. 1, the contour of the ECM is formed by a cylindrical capsule 61. A sound wave passing opening 610 is formed in a front plate 61a of the capsule 61. The following are incorporated into the capsule 61 and arranged in the following order from an inner surface of the front plate 61a toward the rear of the capsule 61: a diaphragm 62, an insulating spacer 63, a rear pole 64, a ring-like rear pole holder 65 consisting of an insulating material, a conductive cylinder 66, and a circuit substrate 67. In this case, the diaphragm 62 comprises a dielectric film which consists of for example, polyphenylene sulfide (also referred to as PPS) and in which a metal film such as Ni or Al is formed, as a conductive layer, on a surface of the film located closer to the rear pole. A diaphragm ring 62a is fixed to the periphery of a front surface of the diaphragm 62 and is in contact with the front plate 61a. The rear pole 64 is placed behind the diaphragm 62 via the thickness of the insulating spacer 63 and supported by the ring-like rear pole holder 65, consisting of an insulating material. A conductive cylinder 66 is interposed between the rear pole 64 and the circuit substrate 67 to electrically connect the rear pole 64 to wiring formed on a top surface (front surface) of the circuit substrate 67. An electret layer 64a is formed on a front surface of the rear pole 64, that is, the surface of the rear pole 64 located opposite the diaphragm 62; the electret layer 64a is obtained by converting a dielectric layer such as FEP (Fluorinated Ethylene Propylene) into an electret. A circuit device 68 such as an FET (Field Effect Transistor) is mounted on a top surface of the circuit substrate 67. Solder bump electrodes 69a and 69b that are externally connected electrodes are projected from a bottom surface (rear surface) of the circuit substrate 67. For example, such a circuit as shown in FIG. 2 is formed on the circuit substrate 67. In FIG. 2, a gate of the FET is connected to the rear pole 64 through the conductive cylinder 66, shown in FIG. 1. A source of the FET is connected to the diaphragm 62 through the capsule 61, shown in FIG. 1. Two capacitors C are connected to between a source and a drain of the FET in parallel with each other; the part between the source and drain of the FET operates as an impedance converting section. The drain of the FET is connected to an output terminal 72 (in FIG. 1, the solder bump electrode 69b) through a through-hole (not shown in the drawings) formed in the circuit substrate 67. The drain of the FET then leads to a DC inhibiting capacitor Cp. The source of the FET is connected to a ground terminal 71 (in FIG. 1, the solder bump electrode 69a) through a through-hole (not shown in the drawings) formed in the circuit substrate 67. Further, the drain of the FET is connected to a reference power source through a resistance element R. In FIG. 1, a rear (back face-side) end of the capsule 61 is caulked to the rear surface of the circuit substrate 67 as a caulking portion 611. The caulking allows element parts housed in the capsule 61 to be fixed to one another. If a sound wave enters the capsule 61 through the sound wave passing opening 610, it vibrates the diaphragm 62 to change the capacitance between the diaphragm 62 and the rear pole 64. This converts the sound wave into an electric signal, which is output to the output terminal 72 (in FIG. 1, the solder bump electrode 69b).
To mount the above ECM on a mounting substrate (not shown in the drawings), the solder bump electrodes 69a and 69b are soldered to the corresponding electrodes on the mounting substrate. That is, the ECM placed on the entire mounting substrate is passed through a reflow bath and then heated. The heating melts the solder bump electrodes 69a and 69b to achieve soldering. In this case, as shown particularly in FIG. 1, the solder bump electrodes 69a and 69b are projected from the bottom surface of the circuit substrate 67, with the caulking portion 611 present on the bottom surface of the circuit substrate 67 at an end of the capsule 61. This configuration presents the problem described below. When the solder is heated and melted in the reflow bath, solder melting heat distorts the caulking portion 611. This may relax the caulking or cause the molten solder and fluxes to advance between the caulking portion 611 and the circuit substrate 67. This may make the electric connection between the rear pole 64 and the wiring on the circuit substrate 67 unstable; the conductive cylinder 66 is interposed between the rear pole 64 and the wiring. The electret layer 64a of the rear pole 64 may be degraded to reduce the voltage applied to between the diaphragm 62 and the rear pole 64. Further, the sensitivity of the ECM may decrease.
With the reflow type ECM for which soldering is carried out using a reflow bath, the measure described below is taken to prevent solder or fluxes from advancing between the caulking portion 611 and the circuit substrate 67. If the mounting substrate is directly soldered, solder paste is accumulated between the caulking portion 611 and the mounting substrate. The caulking portion 611 is thus separated from the mounting substrate before soldering. However, this measure is not reliable.
Another measure involves applying a second substrate to the bottom surface of the circuit substrate 67 to form such a step as projects beyond the thickness of the caulking portion 611. A solder bump electrode is then projected from the substrate. Then, the solder is connected to the mounting substrate in the reflow bath. This amounts to the application of the substrate to the circuit substrate 67 resulting in the formation of a step. The application of the substrate to the circuit substrate 67 requires alignment at a predetermined accuracy and the formation of a through-hole for electric connection followed by an attachment operation. However, these operations preclude inexpensive circuit substrate from being obtained. Further, even if a circuit substrate is obtained by using a router to carry out machining to form a step, disadvantageously the resulting circuit substrate is not inexpensive. That is, structures with steps are expensive.
Moreover, conventional circuit substrates are mostly pattern wired substrates. Fabrication of a pattern wired substrate requires production of conductor electrodes, glass, multilayer wiring, through-holes, and the like using various materials and various printing processes. Consequently, the fabrication process is complicated and expensive.